Certain water-soluble herbicides (e.g., salts of glyphosate, 2,4-D, dicamba, triclopyr, imazapyr, etc.) are useful for suppressing the growth of or killing unwanted woody plants (trees, shrubs, brush, etc.). These herbicides may be formulated in aqueous media that are effective when applied to foliage, but they tend not to give the desired herbicidal effectiveness when applied to bark, such as on dormant woody stems or near the base of dormant or non-dormant woody plants. It is believed that the failure of these aqueous applications is a product of inadequate penetration of the herbicide through the highly hydrophobic or water-repellent outer layers of the bark.
Through-bark applications are, nevertheless, desired methods of herbicidal treatment. The bark near the base of a woody plant may prove more accessible and readily covered than the foliage of such a plant. Through-bark application also permits control of deciduous woody species during winter or other months of sparse foliage, thereby extending the useful season for herbicidal treatment.
One approach that is prevalent in the art has been to convert the water-soluble herbicide into an oil-soluble form, such as an ester, and apply it to the bark in an oil-based carrier. For example, the herbicide triclopyr has been introduced as the 2-butoxyethyl ester in combination with an oil carrier, sometimes referred to as "basal oil." Attempts have also been made to introduce water-soluble herbicides such as salts in oil-based carriers, or in admixture with oil-based adjuvants. But the physical incompatibility between these ingredients makes it difficult to obtain a homogeneous mixture. Even with emulsifiers, it may be difficult to make a storage stable composition containing a water soluble herbicide and sufficient oil to give the desired herbicidal effectiveness when the herbicidal composition is applied to bark.
Aqueous herbicidal formulations are desirable, both for their ease of handling and for their benign environmental impact (as compared to oil-based formulations). The present invention provides a surfactant composition that permits the use of aqueous herbicidal formulations in woody stem applications. This is accomplished through the use of certain organosilicone surfactants, in combination with glycol or glycol ester (and possibly other) materials.
Water-soluble herbicides, such as glyphosate salts, have typically been formulated with surfactants for foliar application, so that when water is added, the resulting sprayable composition will more easily and effectively cover the foliage (e.g., the leaves or other photosynthesizing organs) of plants. Thus, for example, glyphosate salts have been formulated with surfactants such as polyoxyalkylene-type surfactants including, among other surfactants, polyoxyalkylene alkylamines. Commercial formulations of glyphosate herbicide marketed under the trademark ROUNDUP.RTM. have been formulated by Monsanto with such a polyoxyalkylene alkylamine, in particular a polyoxyethylene tallowamine, identified as MON 0818.
It is also known to formulate glyphosate herbicide with polyoxyalkylene polysiloxane surfactants, in particular polyoxyethylene trisiloxane surfactants, such as the commercial organosilicone surfactant Silwet.RTM. L-77, available from OSi Specialties, a group of Witco Corporation. Among the numerous studies of the foliar uptake of glyphosate herbicide combined with Silwet.RTM. L-77 are those reported by Field & Bishop in Pesticide Science, 1988, Vol. 24, pp. 55-62; Stevens et al. in Pesticide Science, 1991, Vol. 33, pp. 371-82; Gaskin & Stevens in Pesticide Science. 1993, Vol. 38, pp. 185-92; and Gaskin & Stevens in Pesticide Science, 1993. Vol. 38, pp. 193-200. An extensive review of 160 references relating to the use of organosilicones as adjuvants for agrochemicals was provided by Stevens in Pesticide Science, 1993, Vol. 38, pp. 103-22. In fact, so many studies are reported in this area that OSi Specialties has published a Bibliography of Silwet.RTM. Organosilicone Surfactants As Agricultural Adjuvants (1996), which is indexed for computer searching. This reference lists hundreds of published studies of commercial organosilicone surfactants in agricultural applications. This bibliography is available to the public through the publisher's office in Tarrytown, N.Y.
In contrast to the vast literature relating to foliar applications of aqueous herbicidal compositions such as those containing glyphosate, information relating to glyphosate application to the basal and dormant stem portions of plants is sketchy. Typically, through-bark, penetration of woody stems has been accomplished by formulating water-insoluble herbicidal materials with oils. In contrast to the oil-based stem treatments of the prior art, a herbicidal composition of the present invention is substantially aqueous or water-miscible or water-soluble, thereby avoiding the expense and environmental impacts of the oil-based systems. The present invention is especially useful in the control of bushy plants such as salt cedar, alder, Brazilian pepper, mesquite, red maple, hickory, and greenbriars. It also permits application of herbicide during winter months, when the species to be controlled may have lost their foliage. It can readily be appreciated that the herbicide uptake behavior for plant stem tissue is dramatically different from that for foliar tissue. Thus different techniques and approaches to enhancement of uptake are warranted. It has been found that the surfactant composition of the present invention is especially useful for this purpose.